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Review Article JOJ Hortic Arboric Volume 1 Issue 3 - May 2018 Copyright © All rights are reserved by Ana Cristina Gonçalves DOI: 10.19080/JOJHA.2018.01.555564 Stand Structure Alterations in Stands

Ana Cristina Gonçalves* Department of Rural Engineering, University of Évora, Portugal Submission: February 15, 2018; Published: May 02, 2018 *Corresponding author: Ana Cristina Gonçalves, Department of Rural Engineering, School of Science and Technology, Institute of Agrarian and Mediterranean Environmental Sciences (ICAAM), Institute of Advanced Research and Training, University of Évora, Portugal, Tel: ; Fax: +351266760911; Email:

Abstract

(pure or mixed). Through time there were shifts between the preferred stand structure frequently due to the desired productions; coppices for smallForest dimension stands are timber usually and classified high forest according for large to dimensionregime (high ones, forest pure or even-aged coppice), structurefor a more (even-aged uniform timber or uneven-aged) dimensions and and composition quality and mixed or uneven-aged for when both timber and services were the target production. The alteration of regime (conversion of regime), structure (conversion of structure) and composition (transformation) has a long history in . Overall, all regimes, structures and compositions contribute to the sustainability and diversity of the forest and their products and services. The alteration of stand structure varies from simple to

enable adapting the silvicultural practices, especially during the transition phase from one stand structure to another. very complex and from short to long term. The focus should be directed to a suite of guidelines, flexible models of and monitoring to Keywords: Conversion of regime; Conversion of structure; Transformation; Transition phase; Models of silviculture

Introduction analysis is described in nearly all silvicultural text books [5- The vast forest areas have provided several products 9,23-24] . It is used not only to describe the forest stands but and services [1,2] and their management varies from also for their management, whether the stands’ production is overexploitation to of abandonment [3,4]. The development of timber, non-timber products or services [9,23,25-28]. According silviculture occurred simultaneously with the shortage of forest to several authors [23-24,29] the variability of stand structure products and was focused on providing and guided to the is wide and as it is determined by the ’s neighbours and even-aged systems [1,5-9]; coppice systems for small dimension their interactions. It is primordial to the stand development wood [7] and high forest for large dimension wood [5-9]. and dynamics, namely growth, mortality, silvicultural practices, Meanwhile the reduction of the pressure on woody products harvests and regeneration. The variability of stand structure is and the environmental concerns altered the focus to broader also associated to some degree heterogeneity, both within and production objectives that included timber, non-timber products between stands, linked with the range of tree dimensions, their and services. The paradigm’s shift directed silviculture to new spatial arrangements in the horizontal and vertical planes and approaches where the emphasis was focused in multiple use systems and uneven-aged and mixed stands and the emulation as the spatial and temporal arrangement of the in a stand of natural processes. This resulted in the development of many the number of species [23,30]. Stand structure can be defined approaches, methods and techniques were developed [1,10-22] . composition [5-9,23-24]. The variation of products and services expected from and is classified with three criteria; regime, structure and Regime of a stand is characterised by the type of regeneration: the created the need to develop methods, techniques high forest for the stands where regeneration is of seed origin and tools to enable the stand structure alteration. In the next and coppice for those of vegetative origin [5,7,24]. However, sections it will be described and discussed the criteria and there is a continuum from high forest to coppice (Figure 1) i. e, in the same stand can exist individuals of both seed and vegetative stand structure, as well as the main forces that drove the shifts definitions used to classify (section 2) and to alter (section 3) origin, thus originating intermediate forms, from the coppice between the different stand structures. with standards to the high forest with coppices. The threshold is Stand Structure set for the proportion of the number of individuals of each of the Forest stands are a group of trees linked by a set of patterns two types of regeneration [7]. Regime is also dependent on the and interactions that originate the stand structure, which species, while high forest is suited for all forest species, coppice was determined by the past processes and where the ongoing can only be used with species that are able to sprout from stumps processes determine the future one. Stand structure and their or roots [7,31] (Figure 1).

JOJ Hortic Arboric 1(3): JOJHA.MS.ID.555564 (2018) 0061 JOJ &

Figure 1: Representation of regime variability. Coppice stands and forests have been a traditional a strong impact on the landscape and high exports of nutrients. management system in Europe [8,32,33]. The coppice system The coppices are frequently referred as having less diversity abandonment started in the late 19th century and was especially due to the spatial arrangement of the trees, as the stands are noticeable during the 20th century [33-36]. Inversely, from the single layered, and due to the clear cuts at short time cycles [41]. late 20th century onwards there was a renewed interest on However, their diversity is distinctive and seems to be related to the coppice systems, especially with the increase importance the coppice management system, short rotations (5-30 years), of small dimension wood for and energy [37,38] and i.e., and high spatial heterogeneity, host a wide range of species their conservation values [38-40]. The success of coppices, that are not present in high forest systems, and have higher when compared with the high forest, can be explained by the resilience to water stress [38,50]. advantages that it brings to the producers, namely, simple Structure is characterised by the number of age classes or management, easy and quick regeneration, fast growth, higher cohorts in the stand, varying from even-aged, one cohort to biomass production and C sequestration and thus short uneven-aged or multiaged, two or more cohorts [22,23,29] rotations resulting in a more frequent income [36,38,41-44]. . At the even-aged extreme are the clonal while at Nonetheless same negative impacts are pointed out by some the uneven-aged are the Plenterwald (Figure 2), in between a authors [45-49], such as soil erosion when clear cuts are used in continuum can be described, at least theoretically, as function of large areas with steep slopes, removal of the soil organic horizon the number of cohorts and their proportion in the stand (Figure by the machinery, cuts of regular areas in large areas may have 2).

Figure 2: Representation of structure variability.

How to cite this article: Gonçalves A C. Stand Structure Alterations in Forest Stands. JOJ Hortic Arboric. 2018; 1(3): 555564. 0062 DOI: 10.19080/JOJHA.2018.01.555564 JOJ Horticulture & Arboriculture

Uneven-aged stands are considered to have higher diversity continuous recruitment, function of the number of cohorts; and than the even-aged ones, due to the wider range of tree ages and an equilibrium for the proportion of the number of individuals dimensions, spatial distribution in the horizontal and vertical planes, and are also considered to be able to have higher value [22,29]. The primordial constraint to their management is their per cohort, frequently determined by the Liocourt coefficient for soil and water conservation and habitat diversity [51-53]. wide range of variability [51].

Composition refers to the number of species present in of silvicultural operations, harvest, products uniformity and Conversely, even-aged stands are driven towards the efficiency forest stands, either pure or mixed [6,8,24] Theoretically, it can predictability [5-9,23-24,51]. Though uneven-aged silviculture be said that it goes from 1 to n species, with all the individuals dates back to the late 18th century, it gained renewed interest belonging to the same species or with each individual belonging with the increasing awareness for diversity and sustainability to a different species (Figure 3). Nonetheless, usually the [22,29]. The uneven-aged systems are characterised by the managed multispecies stands have 2 to 10 species, though larger number of cohorts, target equilibrium of the number of number of species can be found in some natural forests [54-57]. individuals per cohort and regeneration, preferentially natural, Composition is generally evaluated with an absolute density and recruitment able to maintain it [12-13,22,29]. According measure (number of trees, basal area, volume or crown cover), to [29] there should be made a distinction between the canopy and the major challenge is selecting the better suited measure heterogeneity and the silvigenic functionality. Some even-aged and the threshold to distinguish between pure and mixed stands, stands may have a heterogeneous canopy but they are not uneven- being also of interest the form, type and degree of the mixture aged. The latter encompass two or more cohorts; periodical or [9,58,59] (Figure 3).

Figure 3: Representation of composition variability.

Mixed stands have a wide variability, function of the number by their description of the stand structure using absolute density of tree species, their proportion and spatial arrangement [5- measures, [5,6,7,8,9,24] structure indices [7,9,22-24,82-87] and 9,24,56,60]. When compared with the pure stands, have a wider diversity indices [30,62-68,88-98]. In general, it can be said range of productions; have higher resilience to disturbances; that a particular stand structure is better suited to a certain positive interactions are usually found in the stands, in particular production. For example, pure even-aged coppices are preferred when the complementary and sociability principles are met; when small dimension wood is the target production while have higher biodiversity; and encompass more risk dispersion mixed uneven-aged stands are desired when timber and services [23-24,29,56-57,61-71]. The productivity of mixed stands seems are the target (Figure 4). This need adds new challenges to forest to be linked to the number of species and their proportions [72- management and silviculture which encompass a set of methods 74] with some authors referring that they are more productive and tools that enable the alteration of regime, structure and [5,70,75-77] and others stating the opposite [78-81]. of silviculture to promote and accomplish the alterations (Figure Stand structure originates a suite of horizontal and vertical composition, resulting in transitions periods and flexible models 4). distributions of the trees in a stand, which are frequently analysed

How to cite this article: Gonçalves A C. Stand Structure Alterations in Forest Stands. JOJ Hortic Arboric. 2018; 1(3): 555564. 0063 DOI: 10.19080/JOJHA.2018.01.555564 JOJ Horticulture & Arboriculture

Figure 4: Relation between stand structure, productions and services.

Alterations of Stand Structure consider the type, form and degree of mixture between species. Alteration can be an abrupt or gradual phenomenon, if it occurs The alterations of stand structure can be grouped in the during short or long time periods, respectively. The duration following classes: conversion of regime, the alteration from of the transition period from one stand structure to another a coppice to a high forest regime or the inverse, being also depends on the species, their growth rate, type of regeneration considered intermediate forms with individuals of both coppice and their development patterns and site quality. In this phase and high forest; conversion of structure, the alteration from an even-aged to uneven-aged, that is from one cohort to two or the alteration there should be considered its advantages and more cohorts or the inverse; transformation, the alteration of production fluctuations are expected and when preparing disadvantages, including at risk analysis [22,29]. composition from pure to mixed, that is from one species to two or more, or the inverse. Conversation of regime The alteration of stand structure is a process focused The conversion of regime (Figure 5) to high forest occurred on cutting operations and regeneration, where stands are principally when small dimension wood demand diminished and managed towards the target stand structure, regardless whether biodiversity and sustainability issues arose Mariotti et al. [41,99] while the conversion from high forest to coppice was driven by horizontal and vertical, should be taken into consideration due to the increase of demand of small dimension wood, especially for regeneration is natural or artificial. Also, spatial distribution, both its implications in the future management. While the conversion of regime or structure considers the spatial arrangement per issues become of importance [37,38,39,40] (Figure 5). biomass and energy, as well as when some specific conservation regeneration type or cohorts the transformations have also to

Figure 5: Representation of the conversion of regime.

The conversion of regime from high forest to coppice can be i. , dividing the management unit in carried out by a single cut, clearcutting; or a set of cuts, whether a compartments, where each one corresponds to an even-aged variant of clearcutting system or with the shelter wood systems. structure, but where at the management unit several age [34] Consider three methods: classes are represented.

How to cite this article: Gonçalves A C. Stand Structure Alterations in Forest Stands. JOJ Hortic Arboric. 2018; 1(3): 555564. 0064 DOI: 10.19080/JOJHA.2018.01.555564 JOJ Horticulture & Arboriculture

ii. The management unit is divided in compartments; protection more or less heavy, depending on the species and some are maintained as high forest while others are clear cut using a regular pattern. gradually the coppice according to the need in light and site, by , secondary cuts and final cut, removing protection of the plantation. iii. Similar to the former but the areas of coppice have irregular and scatter spatial distribution. iv. Clear cutting with lateral protection, correspond to clear cuts in narrow bands followed by plantation. The key elements of the conversion of regime from coppice to high forest are the cutting operations and the regeneration. The conversion of the with natural regeneration Mariotti et The latter can be natural if seed production is foreseen abundant al. [41] can be done with: and frequent and is of good quality for the desired species. i. from below, selecting at least three dominant sprouts per stump; and scarce in quantity and/or quality, is infrequent or if it does not Contrariwise, artificial regeneration is used when seed source is exist for the desired species [7,8]. It is a more or less complex ii. Selective thinning, selecting the best 1 or 2 sprouts per stump and releasing their crowns. The constraints associated practices [7], but due to the variability of the stands, species and with this type of conversion seem to be related to fruiting process, which can be accomplished by a set of specific cultural and root sprouting [36,100,101]. sitesThe should conversion be flexible of to coppices enable adapting in high management forest stands [41]. with Conversation of structure In the conversion of structure, the cutting operations artificiali. regenerationClear cutting can followed be group inby fourplantation, types [7]: with non- ( and cuttings) and the regeneration are distributed commercial thinning if necessary, and is indicated for robust in time in order to promote the development of one or several and shade intolerant species with fast juvenile growth. cohorts (Figure 6). Special care should be given to the number of cohorts, their spatial distribution, both horizontal and vertical, ii. Cut with reserve of sprouts, followed by plantation and and competition among individuals. The goal is to attain a stand the upper layer composed by the sprouts is removed in 1-3 in equilibrium for the desired number of cohorts. It results in a cuts according to the need of direct sun light and protection drastic change of structure [22,29,51]. It should also be bared of the planted species, being recommended for shade intolerant species that require slight temporary protection. two or three cohorts are similar to those of more cohorts, and in mind that the benefits attained with uneven-aged stands with iii. Cut of protection in the coppice where rotation is enlarged by 25-30 years or more, followed by a cut of (Figure 6). which have an easier and more cost-efficient management [29]

Figure 6: Representation of the conversion of structure and transformation.

The conversion of uneven-aged in even-aged stands can The conversion of even-aged in uneven-aged stands be achieved by one or a set of cutting operations or natural increases in complexity with the increase of the number of disturbances that remove the former stand, so that the cohorts, as recruitment has to be guaranteed periodically (the regeneration occurs mainly at a short time interval and thus shorter the higher the number of cohorts) and it is focused on natural regeneration and on auto regulation mechanisms. The regeneration [23]. conversion of an even-aged stand starts with their evaluation originating a single cohort, attained by natural or artificial How to cite this article: Gonçalves A C. Stand Structure Alterations in Forest Stands. JOJ Hortic Arboric. 2018; 1(3): 555564. 0065 DOI: 10.19080/JOJHA.2018.01.555564 JOJ Horticulture & Arboriculture and encompasses a set of hierarchical efforts to assure: the stand ii. Decide whether to start conversion in the actual stability, the life span of the trees to be maintained during the generation or in the next, depending on the vigour and life transition phase, differentiated regeneration that enables the span of the trees of the over storey. These tw rules can also auto regulation, and the suitability of the uneven-aged archetype be applied to uneven-aged stands with less cohorts than [12,29]. The selection of the number of cohorts is of the the Plenterwald. Overall there is a wide range of structures, utmost importance, and whenever stands develop two or more narrower for even-aged and broader for uneven-aged unbalance cohorts are a good starting point [24]. [12] Considers stands. As a consequence, the models of silviculture should three different alternatives to the conversion to the Plenterwald: classical Plenter thinning, conversion in the actual generation structure to the other. In Figure 7 are presented generic be flexible, especially during the transition phase from one and conversion in the next generation. The option is dependent models of silviculture for the conversion of structure. It is on the level of differentiation of the stand (decreasing from the considered that the starting point (t=0) corresponds to the span of the trees. The same author considers two rules: should then be adapted according to the desired conversion first to the last), the presence or not of regeneration and the life moment of the first evaluation of the stand. The scheme i. Give enough time to the process, with the following of structure, setting the silvicultural practices, their timing stages of: differentiation, promotion, structural development and sequence (Figure 7). and structure achievement

Figure 7: Model of silviculture for the conversion of structure from even-aged to uneven-aged without (top left) and with (top right) change of species and from uneven-aged to even-aged without (bottom left) and with (bottom right) (where CEV is control of spontaneous vegetation, P pruning, T thinning, C cuts, R regeneration, FC final cut, Sp1 species 1, Sp2 species 2, t time, i, k, n moments in time where cultural practices are needed, light grey arrow even-aged structure, dark grey arrow uneven-aged structure).

How to cite this article: Gonçalves A C. Stand Structure Alterations in Forest Stands. JOJ Hortic Arboric. 2018; 1(3): 555564. 0066 DOI: 10.19080/JOJHA.2018.01.555564 JOJ Horticulture & Arboriculture

Transformation rules are similar to those used in the conversion of structure, The transformation of pure stand in mixed ones is focused in stand structure significantly. In this alteration, the criteria and though with some adaptations. Thus, according to the stage of the regeneration of the species to be introduced in the stand, their structural differentiation of the stand the stand stability, the life proportion and spatial arrangement. Of primordial importance span of the trees to be maintained during the transition phase, to the maintenance of mixed stands is also the phenomenon of differentiated regeneration that enables the auto regulation and compatibility, complementarity, forest tree shade tolerance, suitability to the form, type and degree of the desired mixture. By analogy to the conversion of structure, the rules to be followed of each species [22,29]. Conversely, transformation of mixed intra and inter specific competition and the growths patterns are: stands in pure ones can be made selecting the desired species and promoting a set of cuts to remove more or less gradually the i. Give enough time to the process, with the following other species and promoting the regeneration and recruitment stages of differentiation, promotion, structural development and of the desired species. It can be carried out both on even-aged mixture achievement; and on uneven-aged stands. ii. Decide whether to start conversion in the actual In the transformation the initial phases are critical to reach generation or in the next, depending on the vigour and life span the desired form and degree of mixture and the silvicultural of the trees of the over storey. practices, especially the thinning, cuts and regeneration curtail

Figure 8: Model of silviculture for the transformation of a pure in mixed (top left) and mixed in pure (top right) even-aged, pure in mixed (bottom left) and mixed in pure (bottom right) uneven-aged stand (where CEV is control of spontaneous vegetation, P pruning, T thinning, C cuts, R regeneration, FC final cut, Sm main species, Ss secondary species, t time, i, k, n moments in time where cultural practices are needed, light grey arrow even-aged structure, dark grey arrow uneven-aged structure).

How to cite this article: Gonçalves A C. Stand Structure Alterations in Forest Stands. JOJ Hortic Arboric. 2018; 1(3): 555564. 0067 DOI: 10.19080/JOJHA.2018.01.555564 JOJ Horticulture & Arboriculture

The transformation of pure in mixed stands comprises a mortality, individual tree growth and interactions between trees set of cuts that are schedule to enable the regeneration of the [23,29]. For the even-aged stands the development patterns are well known contrary to the uneven-aged ones. As a result, the The former is more sensitive as there have to be guaranteed seed conversion of structure can be more or less complex, short or desired species. The regeneration can be natural or artificial. source from the desired species which different ecological and long term lasting, and based on the diversity of management cultural requirements. Seed trees of all the desired species have options [12,13,22,52,53,105,106]. to in or in the neighbourhoods of the area to regenerate so that When compared with the pure stands the mixed ones seed can be dispersed in all the area uniformly. Thus thinning provide more products and services [22,23,57], but are also can be prescribed to promote de seed production and cuts such more complex, especially when they are uneven-aged [16,17,23]. as clear cuttings, shelter wood or selective cuts to remove the The composition complexity is related to the number of species, existing stand and promote the establishment of regeneration. form, type and degree of mixture, density light levels, mortality, the desired species can be seeded or planted in the most suitable In transformation with artificial regeneration after the harvest As a result, management increases in complexity from pure to niches, according to their and the site characteristics. The latter individual tree growth and intra and inters specific interactions. mixed stand and consequently transformation can be more or can be also used to complement the natural regeneration, less complex [12,22,103,104]. when it is scarce or when it is not uniformly distributed. The transformation of mixed in pure stands is implemented essentially From the aforementioned it can be said that all regimes, through thinnings and cuttings, removing the individuals of all structures and compositions contribute to the sustainability species except the desired one. The main goal is removing as early and diversity of the forest, and their products and services. The as possible the undesired species to reduce their regeneration. It conversion of regime, structure and transformation originate more or less long transition phases and can vary from relatively et al. [102]. It can also be considered the conversion of structure simple to very complex. Considering the wide range if stand can be used the natural or the artificial regeneration MacDonald and the transformation simultaneously, though it is considerable structures and site conditions it seems that the focus should criteria and the rules applied for both alteration processes, and monitoring to enable adapting the silvicultural practices, be set on a suite of guidelines, flexible models of silviculture more difficult. The silvicultural operations should follow the especially when the alteration of stand structure is ongoing. aforementioned for the structure there is a wide variability in and seem more difficult to achieve [103,104]. Similarly, to the stand composition, due to the number of species and their form Acknowledgement suited, especially during the transformation. Figure 8 presents Foundation for Science and Technology under the Project UID/ The work was financed by National Funds through FCT- type and degree. Thus, flexible models of silviculture and better AGR/00115/2013. evaluation moment corresponds to the starting point (t=0) generic models do silviculture for the transformation. The first Conclusion and the cultural practices schedule, type and sequence should be adapted to the stand under transformation according to the genetic variability among 6 Bulgarian peanuts varieties 1. The present study identifies the presence of adequate (Arachis hypogaea L.) of the Valencia type. Discussiondefined objectives and (FigureConclusion 8). 2. The information gathered from this study can be used The conversion of regime results in structure and composition for a future program for the selection of peanuts resistant to alterations as well as stand dynamics. The biggest alterations water stress. occur in the vertical plane with variations of the under storey 3. The oldest Bulgarian variety, Sadovo 2609, has been [33]. Considering biodiversity and sustainability there seems to density, of the height of the canopy layer and in the foliage profile be different opinions, but, in general, it can be said that each found4. It to has use been inefficient found water that resourcesgenes expressing in the soil. the ratio of regime has its distinctive features and a combination at landscape water to leaf area units (gH2O/cm2) are homozygous as a level of several regimes with several spatial distribution patterns result of the action of inbreeding. will promote more diversify of the productions and services of the forest. In what regards silviculture, in the conversion of 5. The role of environmental conditions has been regime several methods can be used [7,41]. established for the parameters of water exchange, chlorophyll content and leaf size in peanuts. The primordial difference between even-aged and uneven- aged stands is the variety of structures, more reduced for the 6. The low PCV and GCV values for most indicators show former and broader to the latter [22,23,29]. The uneven-aged that variability is required in them. structures can vary from relatively simple to very complex. 7. The high and moderate GAM values reported in fresh The increase of complexity is linked to the number of cohorts, and dry mass, water content, transpiration, and water-to-leaf the proportion of individuals per cohort, density, light levels, area ratio suggest potential for genetic improvement

How to cite this article: Gonçalves A C. Stand Structure Alterations in Forest Stands. JOJ Hortic Arboric. 2018; 1(3): 555564. 0068 DOI: 10.19080/JOJHA.2018.01.555564 JOJ Horticulture & Arboriculture

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How to cite this article: Gonçalves A C. Stand Structure Alterations in Forest Stands. JOJ Hortic Arboric. 2018; 1(3): 555564. 0071 DOI: 10.19080/JOJHA.2018.01.555564 JOJ Horticulture & Arboriculture

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How to cite this article: Gonçalves A C. Stand Structure Alterations in Forest Stands. JOJ Hortic Arboric. 2018; 1(3): 555564. 0072 DOI: 10.19080/JOJHA.2018.01.555564